1. Field of the Invention
The present invention relates to a method of forming tube blanks to achieve a desired shape. Specifically, the method involves the forming of tube blanks that have a variable wall thickness. More specifically, the forming process comprises hydroforming.
2. Description of the Prior Art
In the automotive industry, various structural components are made from tubular blanks. Such blanks are usually formed into the desired component shapes using various forming technologies. One of such techniques that has become popular is hydroforming. In hydroforming, a tubular blank is placed within a die having a shape of the desired component. The ends of the tube are sealed and a pressurized fluid is applied to the interior of the blank. Such pressure expands the blank until it conforms to the shape of the die cavity. In the usual case, prior to hydroforming, the tube blank is bent to the desired shape and the hydroforming step is used to provide the desired cross sectional shape. Generally, the wall thickness of the tubular blank is generally maintained throughout the forming process, although a slight degree of reduction may be realized due to the stretching of the tube.
In certain cases, it is desired that the finished product or component have a variable thickness in order to, inter alia, reduce the overall weight of the final product or to reduce the cost of the materials used to form the component. In other cases, the final product is required to have localized reinforcing in regions that are subjected to stresses, such as bends etc. or, in other cases, regions of weakness so as to preferably facilitate the bending of such sections. Various methods have been suggested to provide such variable wall product. For example, the method taught in U.S. Pat. No. 5,333,775 involves a number of tubular pieces of different wall thicknesses to be welded together to form the blank used in the hydroforming step. Although resulting in the required variable wall blank, and, therefore, formed product, this method includes various deficiencies. Firstly, the method involves the pre-forming step of creating the multiwall blank using a welding procedure, which adds a considerable amount to the total processing time. Secondly, the presence of welds may lead to weak spots in the formed product.
In U.S. Pat. No. 4,759,111, another method is taught where a first tube, of a constant wall thickness, is provided at certain locations with a co-axial sleeve thereby resulting in a tubular blank with a variable wall thickness. Such blank is then subjected to a hydroforming process as discussed above. Although no welding steps are performed, this method results in a non-homogenous product.
U.S. Pat. No. 5,557,961 teaches a method wherein the tubular blank is provided with a constant outer diameter, and which is used in a hydroforming process. The wall thickness of the blank taught in this reference varies circumferentially but is constant in the longitudinal direction. In other words, the blank is provided with longitudinal grooves along the interior surface. As a result, the component formed according the the '961 method includes thin sections, at any given cross section, wherein such thin sections extend along the length of the component. This reference does not, however, teach a formed component having a constant cross sectional or circumferential wall thickness.
The present invention provides an improved method for providing a formed product having a variable longitudinal wall thickness that overcomes at least some of the deficiencies discussed above.